Clay bleaching



limited States Patent 3,001,852 CLAY BLEACHING Robert F. Conley, Elizabeth, N..l., assignor to Georgia Kaolin Company, Elizabeth, N.J., a corporation of New Jersey No Drawing. Filed May 22, 1959, Ser. No. 814,987 3 Claims. (Cl. 23-110) This invention deals with chemical reactions wherein there is employed at least one unstable reagent which tends to decompose spontaneously in quantity in presence of a material that is improved by the reagent. More specifically, it relates to the addition of said unstable reagent to the reaction mixture in several increments while stirring between said increments without removal of reaction components. Another feature of this invention is the improvement of color and viscosity by addition of synergistic amounts of sodiumbisulfite to certain reagents employed in the aforesaid reactions.

In some reactions involving an unstable reagent, as for example a bleaching reagent, it has been shown in the art that advantage can be gained by successive addition of the reagent to the material being bleached, as compared to a single addition of the complete amount of reagent used. In such cases, each addition of reagent has been succeeded by a washing step which removes the products of reaction as well as excess bleach which may have remained after said addition.

It has been found, in the case of the present invention, that even better results may be obtained in certain bleaching reactions by successive additions of the unstable reagent, each followed by a reaction period in presence of the entire reaction mixture, i.e., without a washing step. When the unstable reagent is added in increments according to the present invention, it is desirable that the final increment be added at a time not over 80%, or less than about 15%, of the total period of treating the material with the unstable reagent, while mixing. In general, best results are obtained in the high range, i.e., 40% to 80%, of the total period. In the case of sodium hypochlorite as the unstable reagent, the final increment is best added in the upper side of the aforesaid range, i.e., around 80%, while in the case of sodium dithionite (hydrosulfite), the addition is best made on the lower side, i.e., around 40%.

Further in the case of sodium dithionite (N21 S O as the reagent, it has been found that use of synergistic amounts (5 to 15% by weight of the total reagent employed) of a second but much weaker reagent, sodium bisulfite, makes it possible particularly when bleaching clays, to reduce slurry viscosity, while at the same time obtaining improvement in color reduction. This effect is more marked when the bisulfie addition is made with the first addition of the reagent.

One example of an unstable reagent is sodium dithionite which decomposes spontaneously in acid solution and which reacts readily with oxygen, whereby it loses its power as a bleaching agent. Sodium dithionite is employed in industry as a bleaching agent for clay in aqueous suspension. It is also employed for bleaching other materials. Another unstable reagent is sodium hypochlorite (NaOCl) which is used extensively for bleaching textiles, paper pulp and other materials.

In the case of the bleaching of clay with sodium dithionite, the highly colored ferric iron present in the clay as impurity is reduced by the reagent to the much lower colored ferrous iron, and the improvement in color of the clay is mainly attributed to this particular reduction reaction.

The invention will be more readily understood from the following examples which bring out some of the many novel phases of the present invention:

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Example 1 A Georgia kaolin was suspended in a mixture containing 65% water which was brought to a pH of 3.4 by sulfuric acid and aluminum sulfate. It was then treated at 25 C. with 15 lb. per ton (on the basis of the clay) of commercial sodium dithionite (10 lb. S0 equivalent) and stirred continuously (and carefully to avoid excessive contact with air) for minutes. A sample of the clay was then removed and found to have a brightness of 84.1, compared to 77.5 in the natural unbleached form, the brightness being measured with the standard acceptable General Electric reflectance spectrometer.

Example 2 A sample of the clay of Example 1 was treated in the same manner, with the exception that 6 lb. per ton of commercial sodium dithionite were first added, while stirring, and the mixture was continuously stirred for 20 minutes. Thereafter, another 6 lb. per ton of the same reagent were added and stirring was continued for another 20 minutes, after which a sample of the clay was removed and found to have a brightness of 85.0.

This brightness is significantly higher than the value of 84.1 figure for the single addition as described in Example 1, even though the bleach content was reduced by 3 lb. per ton.

Example 3 A sample of the clay of Example 1 was made up in the same way as in Example 1, with the exception that the suspension was acidified only with sulfuric acid to a pH value of 3.4. The clay was treated with 7.8 lb. per ton of sodium dithionite in a single step, followed by stirring for 40 minutes. Thereafter, a sample of the clay was withdrawn and found to have a brightness value of Example 4 A sample of the clay of Example 1 was treated in the same manner as in Example 3 with the exception that 3.9 lb. per ton of sodium dithionite were first added and the mixture was stirred for 15 minutes and then treated again with another 3.9 lb. per ton of reagent. After 40 minutes of total bleaching time, while stirring, a sample of clay was withdrawn and found to have a'brightness value of 83.1, which is considerably whiter than the value of 82.0 for the single treatment described in Example 3.

Example 5 Example 6 A sample of the clay of Example 1 was treated in the same manner as in Example 3, with the exception that a single treatment with 12 lb. per ton of reagent was carried out for a total bleach time of 40 minutes. At the end of the bleach period, the clay showed a brightness value of 82.3.

Example 7 A sample of the clay of Example 1 was treated as in Example 6 with the exception that the reagent was added in two increments of 6 lb. per ton, 20 minutes apart, for a total bleaching time of 40 minutes. At the end of this period a sample of the clay was withdrawn and-found '3 to have a brightness value of 84.4, a considerable increase over the value of 82.3 for the single treatment described in Example 6.

Example 8 A sample of clay of Example 1 was treated as in Example 6 except that the reagent was added in 4 increments of 3 lb. per ton, 4 minutes apart with a total bleaching time of 40 minutes. A sample of clay was withdrawn at the end of the 40 minutes and found to have a brightness of 82.7, a value only slightly better than that for a one-batch treatment as described in Example 6.

Example 9 A sample of clay of Example 1 was treated in the same manner as described in Example 1 with the excep tion that 16.5 lb. per ton of the reagent were added in one batch and the mixture was stirred for 40 minutes. A sample of the clay was removed after the 40 minutes and found to have a brightness value of 83.3.

Example 10 A sample of clay of Example 1 was treated as in Example 1 with the exception that lb. per ton of reagent were added in one batch and the mixture stirred for minutes. Thereafter, 1.5 lb. per ton of reagent were added with stirring until a total bleaching time of 40 minutes elapsed, when a sample was withdrawn and found to have a brightness of 84.0, an improvement over the value of 83.3 for the single batch treatment described in Example 9.

Example 11 A sample of Georgia kaolin, such as that used in Example 1, was submitted to bleaching with aqueous sodium dithionite (pH 3.4) in various increments (equally spaced) and the results obtained were as follows:

1 In this run, 1.2 lb. of sodium bisulfite were added at a time not later than the first addition of dithionite. Total bleaching time in each series was 40 minutes.

From the above table, it is apparent that the use of three increments is superior to other types of addition, saving of bleaching agent also being etfected with accompanying increase in brightness. A further feature shown by the above table, as evidenced from run No. 6, is that a small addition (about 10% by weight of total bleach) of sodium bisulfite (a cheaper reagent) effects a decided improvement in color when used with the multi-increment additions of dithionite.

This latter effect of sodium bisulfite on the incremental effect of the reagent results in a further advantage in viscosity decrease of the clay slurry, particularly at high solids contents, which is important in paper-coating where as much clay as is possible is employed in the coating formulation. This improvement is apparent from the succeeding example, No. 12.

Example 12 A number of kaolin clays were treated with sodium .dithionite bleaching reagent in 1- and 3-increment systems, and sodium bisulfite was added during the first addition of reagent in the multi-increment runs, with results obtained as follows:

Total Number of Viscosity,

Clay Amount of Incre- Reflectance ccntipoiscs Reagent, ments at lb./ton r.p.ln.

Example 13 Various cotton cloths, dyed with bleachable dyes, were cut into strips and placed into an aqueous acid solution (pH 1.0), and various increments (equally spaced as to time) of concentrated sodium hypochlorite solution (10 weight percent) were added as follows, the final concentration of the treating solution in each case being 1.26 weight percent:

Time of Total Brightness No. of Addition Treating Gain 1 Series Increments of Last time (Percent of) Increment (Hrs) Original Hrs.)

1 2. 0 0. l 1 l a 2. 0 2. 0 2. 5 2 1 2. 5 0. 3 3 2. 0 2. 5 6. 4 3 1 4. 0 ll. 3 3 2. O 4. 0 l3. 8

1 A blank was run on the acid solution alone and brightness gains were corrected therefor.

It is apparent from the above table that the best gain is obtained as exhibited by the second series wherein the ratio of time of addition of the last increment to the total time was 80%. This has been borne out by numerous other data not submitted herein.

It is apparent that not more than three increments of reagent are to be added to achieve the best results upon the material treated, by the method described herein. It has been found further that the period of reaction between the first addition of reagent and the final addition, should not exceed about 80% of the total bleaching time, and, under the conditions described herein, in no case should it exceed 40 minutes for sodium dithionite. It has been found desirable that the reagent be added in substantially equal batches during substantially equally-spaced time periods.

I claim:

1. The process of bleaching, during a bleaching period in an acid solution, of a clay containing iron coloring impurities capable of being bleached, comprising the steps of first adding only a portion of sodium dithionite not exceeding one-half of the total dithionite used, then subjecting the dithionite and said clay to a mixing operation for a period of not over 80% of the total bleaching period, then adding at least one additional portion and not over two successive portions of dithionite, in amounts substantially equal to that of the first portion, in the presence of substantially the entire bleaching mixture resulting from the preceding dithionite additions, and stirring for the remainder of the bleaching period.

2. The process of bleaching, during a bleaching period in an acid solution, of a clay containing iron coloring impurities capable of being bleached, comprising adding of said clay three separate substantially equal additions of sodium dithionite at substantially equally-timed intervals in the presence of substantially the entire bleaching mixture resulting from preceding dithionite additions, and stirring during the entire bleaching period.

3. The process of bleaching, during a bleaching period in an acid solution, of a clay containing iron coloring impurities capable of being bleached, comprising the steps of first adding only a portion of sodium dithionite not exceeding one-half of the total dithionite used, adding sodium bisulfite in an amount of about 5 to 15% by weight of the total dithionite used while stirring for a period of not over 80% of the total bleaching period, then References Cited in the file of this patent UNITED STATES PATENTS 1,036,831 Gruber Aug. 2, 1912 1,588,956 Feldenheimer June 15, 1926 2,494,542 Casciani et al. Jan. 17, 1950 FOREIGN PATENTS 242,357 Great Britain Nov. 9, 1925 

1. THE PROCESS OF BLEACHING, DURING A BLEACHING PERIOD IN AN ACID SOLUTION, OF A CLAY CONTAINING IRON COLORING IMPURITIES CAPABLE OF BEING BLEACHED, COMPRISING THE STEPS OF FIRST ADDING ONLY A PORTION OF SODIUM DITHIONITE NOT EXCEEDING ONE-HALF OF THE TOTAL DITHIONITE USED, THENSUBJECTING THE DITHIONITE AND SAID CLAY TO A MIXING OPERATION FOR A PERIOD OF NOT OVER 80% OF THE TOTAL BLEACHING PERIOD, THEN ADDING AT LEAST ONE ADDITIONAL PORTION AND NOT OVER TWO SUCCESSIVE PORTIONS OF DITHIONITE, IN AMOUNTS SUBSTANTIALLY EQUAL TO THAT OF THE FIRST PORTION, IN THE PRESENCE OF SUBSTANTIALLY THE ENTIRE BLEACHING MIXTURE RESULTING FROM THE PRECEDING DITHIONITE ADDITIONS, AND STIRRING FOR THE REMAINDER OF THE BLEACHING PERIOD. 